A head lamp in a vehicle is considered a small size component but the headlamp can be of great importance with regard to function and design. In a functional view, the headlamp is basically for enhancing a driver's view and informing a vehicle's existence to other drivers or pedestrians. Further, since different types of vehicle styling can be achieved by the head lamps, as well as front ventilation grilles, the headlamps can be recognized as an important ornamental and a functional component.
As the headlamp, an incandescent lamp having a filament was used in the past. However, such an incandescent lamp has limits because of its short life and low luminance. After the incandescent lamp, a halogen lamp which is a kind of an incandescent lamp that has a small amount of a halogen was used as a headlamp of vehicles because the halogen lamp can emit a wide diffusion of light, can be easy to enhance a driver's view, and can be miniaturized or downsized. Further, in order to satisfy customer needs for a high-intensity, high-efficiency headlamp, a manufacturer can use a high-intensity discharge (HID) lamp for vehicle's headlamps.
Most vehicles are equipped with a lamp having a filament. However, if turned off, the temperature of the filament falls. As the temperature falls, the filament's resistance becomes lower. Thus, when the lamp is turned on after completely cooling off, an inrush current having a magnification of 1,300% a rated load for a very short time such as about 100 ms can flow in a moment throughout the filament in the lamp. The inrush current can shorten the headlamp's life. In order to overcome an issue of shortening the headlamp's lifespan by the inrush current, it is suggested that a wire in the lamp is made thicker. However, this suggestion can bring another issue of increasing a lamp's weight and a manufacturing cost.